serial: stm32: remove bits related to !HAS_DTS

Now that all STM32 platforms are using device tree we can remove the
handling for !HAS_DTS from the serial driver.

Change-Id: Ifafc283f2509dd9a438f321e0b647720d4f13810
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>